1. Field of the Invention
The present invention relates to an ultrasonic motor.
2. Description of Related Art
Recently, ultrasonic motors have been drawing attention as a new type of motor replacing electromagnetic motors. Ultrasonic motors have the following advantages over known electromagnetic motors:
1) Ultrasonic motors are capable of high torque without using gears;
2) Ultrasonic motors have holding force when powered off;
3) Ultrasonic motors have high resolution;
4) Ultrasonic motors are quiet; and
5) Ultrasonic motors do not generate magnetic noise and are unaffected by noise.
A known ultrasonic motor is described in Japanese Unexamined Patent Application Publication No. 2001-258277. The ultrasonic motor disclosed in Japanese Unexamined Patent Application Publication No. 2001-258277 has a construction including at least three driving units on the surface of an elastic body opposing a driven body.
With the ultrasonic motor disclosed in Japanese Unexamined Patent Application Publication No. 2001-258277, all of the driving units exhibit circular or elliptical motion in the same directions, and to finely (that is, minutely) move the driven body, it is thus necessary to reduce the vibration amplitude of the circular or elliptical motion. However, it is difficult to control the vibration amplitude of the elastic body while maintaining resonance, and as a result, there is the drawback that it is difficult to achieve stable, fine motion of the driven body.